Micro-electromechanical system based arc-less switching with circuitry for absorbing electrical energy during a fault condition

ABSTRACT

A system is presented. The system includes a micro-electromechanical system switch. Further, the system includes a balanced diode bridge configured to suppress arc formation between contacts of the micro-electromechanical system switch. A pulse circuit is coupled to the balanced diode bridge to form a pulse signal in response to a fault condition. An energy-absorbing circuitry is coupled in a parallel circuit with the pulse circuit and is adapted to absorb electrical energy resulting from the fault condition without affecting a pulse signal formation by the pulse circuit.

RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 11/314,336 filed on Dec. 20, 2005, which isincorporated by reference in its entirety herein.

BACKGROUND

Embodiments of the invention relate generally to a switching device forswitching off a current in a current path, and more particularly tomicro-electromechanical system based switching devices.

A circuit breaker is an electrical device designed to protect electricalequipment from damage caused by faults in the circuit. Traditionally,most conventional circuit breakers include bulky electromechanicalswitches. Unfortunately, these conventional circuit breakers are largein size thereby necessitating use of a large force to activate theswitching mechanism. Additionally, the switches of these circuitbreakers generally operate at relatively slow speeds. Furthermore, thesecircuit breakers are disadvantageously complex to build and thusexpensive to fabricate. In addition, when contacts of the switchingmechanism in conventional circuit breakers are physically separated, anarc is typically formed therebetween which continues to carry currentuntil the current in the circuit ceases. Moreover, energy associatedwith the arc may seriously damage the contacts and/or present a burnhazard to personnel.

As an alternative to slow electromechanical switches, fast solid-stateswitches have been employed in high speed switching applications. Aswill be appreciated, these solid-state switches switch between aconducting state and a non-conducting state through controlledapplication of a voltage or bias. For example, by reverse biasing asolid-state switch, the switch may be transitioned into a non-conductingstate. However, since solid-state switches do not create a physical gapbetween contacts when they are switched into a non-conducing state, theyexperience leakage current. Furthermore, due to internal resistances,when solid-state switches operate in a conducting state, they experiencea voltage drop. Both the voltage drop and leakage current contribute tothe generation of excess heat under normal operating circumstances,which may be detrimental to switch performance and life. Moreover, dueat least in part to the inherent leakage current associated withsolid-state switches, their use in circuit breaker applications is notpossible.

BRIEF DESCRIPTION

Briefly, in accordance with aspects of the present technique, a systemis presented. The system includes a micro-electromechanical systemswitch. A balanced diode bridge is configured to suppress arc formationbetween contacts of the micro-electromechanical system switch. A pulsecircuit is coupled to the balanced diode bridge. The pulse circuitcomprises a pulse capacitor adapted to form a pulse signal for causingflow of a pulse current through the balanced diode bridge. The pulsesignal is generated in response to a fault condition in a load circuitcoupled to the micro-electromechanical system switch. Anenergy-absorbing circuitry is coupled in a parallel circuit with thepulse circuit. The energy-absorbing circuitry comprises anenergy-absorbing capacitor adapted to absorb electrical energy resultingfrom the fault condition without affecting a pulse signal formation bythe pulse circuit.

In accordance with further aspects of the present technique a system ispresented. The system includes switching circuitry comprising amicro-electromechanical system switch configured to switch the systemfrom a first switching state to a second switching state. An arcsuppression circuitry is coupled to the switching circuitry, wherein thearc suppression circuitry is configured to suppress an arc formationbetween contacts of the micro-electromechanical system switch. Detectioncircuitry is coupled to the arc suppression circuitry and configured todetermine existence of a fault condition. A pulse circuit is coupled tothe arc suppression circuitry and the detection circuitry, wherein thepulse circuit is configured to form a pulse signal responsive to thefault condition, and wherein the pulse signal is applied to the arcsuppression circuitry in connection with initiating an opening of themicro-electromechanical system switch. An energy-absorbing circuitrycoupled in a parallel circuit with the pulse circuit. Theenergy-absorbing circuitry is adapted to absorb electrical energyresulting from the fault condition without affecting a pulse signalformation by the pulse circuit.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a block diagram of an exemplary MEMS based switching system,in accordance with aspects of the present technique;

FIG. 2 is schematic diagram illustrating the exemplary MEMS basedswitching system depicted in FIG. 1;

FIGS. 3-5 are schematic flow charts illustrating an example operation ofthe MEMS based switching system illustrated in FIG. 2;

FIG. 6 is schematic diagram illustrating a series-parallel array of MEMSswitches;

FIG. 7 is schematic diagram illustrating a graded MEMS switch;

FIG. 8 is a flow diagram depicting an operational flow of a systemhaving the MEMS based switching system illustrated in FIG. 1;

FIG. 9 is a graphical representation of experimental resultsrepresentative of turn off of the switching system.

FIG. 10 is schematic diagram illustrating an exemplary MEMS-basedswitching system, in accordance with aspects of the present invention;and

FIGS. 11 and 12 respectively illustrate a graphical representation ofsimulation results of example circuit signals illustrative ofoperational details of the switching system of FIG. 10, in accordancewith aspects of the present invention.

DETAILED DESCRIPTION

In accordance with one or more embodiments of the present invention,systems and methods for micro-electromechanical system based arc-lessswitching is described herein. In the following detailed description,numerous specific details are set forth in order to provide a thoroughunderstanding of various embodiments of the present invention. However,those skilled in the art will understand that embodiments of the presentinvention may be practiced without these specific details, that thepresent invention is not limited to the depicted embodiments, and thatthe present invention may be practiced in a variety of alternativeembodiments. In other instances, well known methods, procedures, andcomponents have not been described in detail.

Furthermore, various operations may be described as multiple discretesteps performed in a manner that is helpful for understandingembodiments of the present invention. However, the order of descriptionshould not be construed as to imply that these operations need beperformed in the order they are presented, nor that they are even orderdependent. Moreover, repeated usage of the phrase “in one embodiment”does not necessarily refer to the same embodiment, although it may.Lastly, the terms “comprising”, “including”, “having”, and the like, asused in the present application, are intended to be synonymous unlessotherwise indicated.

FIG. 1 illustrates a block diagram of an exemplary arc-lessmicro-electromechanical system switch (MEMS) based switching system 10,in accordance with aspects of the present invention. Presently, MEMSgenerally refer to micron-scale structures that for example canintegrate a multiplicity of functionally distinct elements, e.g.,mechanical elements, electromechanical elements, sensors, actuators, andelectronics, on a common substrate through micro-fabrication technology.It is contemplated, however, that many techniques and structurespresently available in MEMS devices will in just a few years beavailable via nanotechnology-based devices, e.g., structures that may besmaller than 100 nanometers in size. Accordingly, even though exampleembodiments described throughout this document may refer to MEMS-basedswitching devices, it is submitted that the inventive aspects of thepresent invention should be broadly construed and should not be limitedto micron-sized devices.

As illustrated in FIG. 1, the arc-less MEMS based switching system 10 isshown as including MEMS based switching circuitry 12 and arc suppressioncircuitry 14, where the arc suppression circuitry 14 is operativelycoupled to the MEMS based switching circuitry 12. In certainembodiments, the MEMS based switching circuitry 12 may be integrated inits entirety with the arc suppression circuitry 14 in a single package16, for example. In other embodiments, only certain portions orcomponents of the MEMS based switching circuitry 12 may be integratedwith the arc suppression circuitry 14.

In a presently contemplated configuration as will be described ingreater detail with reference to FIGS. 2-5, the MEMS based switchingcircuitry 12 may include one or more MEMS switches. Additionally, thearc suppression circuitry 14 may include a balanced diode bridge and apulse circuit. Further, the arc suppression circuitry 14 may beconfigured to facilitate suppression of an arc formation betweencontacts of the one or more MEMS switches. It may be noted that the arcsuppression circuitry 14 may be configured to facilitate suppression ofan arc formation in response to an alternating current (AC) or a directcurrent (DC).

Turning now to FIG. 2, a schematic diagram 18 of the exemplary arc-lessMEMS based switching system depicted in FIG. 1 is illustrated inaccordance with one embodiment. As noted with reference to FIG. 1, theMEMS based switching circuitry 12 may include one or more MEMS switches.In the illustrated embodiment, a first MEMS switch 20 is depicted ashaving a first contact 22, a second contact 24 and a third contact 26.In one embodiment, the first contact 22 may be configured as a drain,the second contact 24 may be configured as a source and the thirdcontact 26 may be configured as a gate. Furthermore, as illustrated inFIG. 2, a voltage snubber circuit 33 may be coupled in parallel with theMEMS switch 20 and configured to limit voltage overshoot during fastcontact separation as will be explained in greater detail hereinafter.In certain embodiments, the snubber circuit 33 may include a snubbercapacitor (not shown) coupled in series with a snubber resistor (notshown). The snubber capacitor may facilitate improvement in transientvoltage sharing during the sequencing of the opening of the MEMS switch20. Furthermore, the snubber resistor may suppress any pulse of currentgenerated by the snubber capacitor during closing operation of the MEMSswitch 20. In certain other embodiments, the voltage snubber circuit 33may include a metal oxide varistor (MOV) (not shown).

In accordance with further aspects of the present technique, a loadcircuit 40 may be coupled in series with the first MEMS switch 20. Theload circuit 40 may include a voltage source V_(BUS) 44. In addition,the load circuit 40 may also include a load inductance 46 L_(LOAD),where the load inductance L_(LOAD) 46 is representative of a combinedload inductance and a bus inductance viewed by the load circuit 40. Theload circuit 40 may also include a load resistance R_(LOAD) 48representative of a combined load resistance viewed by the load circuit40. Reference numeral 50 is representative of a load circuit currentI_(LOAD) that may flow through the load circuit 40 and the first MEMSswitch 20.

Further, as noted with reference to FIG. 1, the arc suppressioncircuitry 14 may include a balanced diode bridge. In the illustratedembodiment, a balanced diode bridge 28 is depicted as having a firstbranch 29 and a second branch 31. As used herein, the term “balanceddiode bridge” is used to represent a diode bridge that is configuredsuch that voltage drops across both the first and second branches 29, 31are substantially equal. The first branch 29 of the balanced diodebridge 28 may include a first diode D1 30 and a second diode D2 32coupled together to form a first series circuit. In a similar fashion,the second branch 31 of the balanced diode bridge 28 may include a thirddiode D3 34 and a fourth diode D4 36 operatively coupled together toform a second series circuit.

In one embodiment, the first MEMS switch 20 may be coupled in parallelacross midpoints of the balanced diode bridge 28. The midpoints of thebalanced diode bridge may include a first midpoint located between thefirst and second diodes 30, 32 and a second midpoint located between thethird and fourth diodes 34, 36. Furthermore, the first MEMS switch 20and the balanced diode bridge 28 may be tightly packaged to facilitateminimization of parasitic inductance caused by the balanced diode bridge28 and in particular, the connections to the MEMS switch 20. It may benoted that, in accordance with exemplary aspects of the presenttechnique, the first MEMS switch 20 and the balanced diode bridge 28 arepositioned relative to one another such that the inherent inductancebetween the first MEMS switch 20 and the balanced diode bridge 28produces a di/dt voltage less than a few percent of the voltage acrossthe drain 22 and source 24 of the MEMS switch 20 when carrying atransfer of the load current to the diode bridge 28 during the MEMSswitch 20 turn-off which will be described in greater detailhereinafter. In one embodiment, the first MEMS switch 20 may beintegrated with the balanced diode bridge 28 in a single package 38 oroptionally, the same die with the intention of minimizing the inductanceinterconnecting the MEMS switch 20 and the diode bridge 28.

Additionally, the arc suppression circuitry 14 may include a pulsecircuit 52 coupled in operative association with the balanced diodebridge 28. The pulse circuit 52 may be configured to detect a switchcondition and initiate opening of the MEMS switch 20 responsive to theswitch condition. As used herein, the term “switch condition” refers toa condition that triggers changing a present operating state of the MEMSswitch 20. For example, the switch condition may result in changing afirst closed state of the MEMS switch 20 to a second open state or afirst open state of the MEMS switch 20 to a second closed state. Aswitch condition may occur in response to a number of actions includingbut not limited to a circuit fault or switch ON/OFF request.

The pulse circuit 52 may include a pulse switch 54 and a pulse capacitorC_(PULSE) 56 series coupled to the pulse switch 54. Further, the pulsecircuit may also include a pulse inductance L_(PULSE) 58 and a firstdiode D_(P) 60 coupled in series with the pulse switch 54. The pulseinductance L_(PULSE) 58, the diode D_(P) 60, the pulse switch 54 and thepulse capacitor C_(PULSE) 56 may be coupled in series to form a firstbranch of the pulse circuit 52, where the components of the first branchmay be configured to facilitate pulse current shaping and timing. Also,reference numeral 62 is representative of a pulse circuit currentI_(PULSE) that may flow through the pulse circuit 52.

In accordance with aspects of the present invention as will be describedin further detail hereinafter, the MEMS switch 20 may be rapidlyswitched (e.g., on the order of picoseconds or nanoseconds) from a firstclosed state to a second open state while carrying a current albeit at anear-zero voltage. This may be achieved through the combined operationof the load circuit 40, and pulse circuit 52 including the balanceddiode bridge 28 coupled in parallel across contacts of the MEMS switch20.

FIGS. 3-5 are used as schematic flow charts to illustrate an exampleoperation of the arc-less MEMS based switching system 18 illustrated inFIG. 2. With continuing reference to FIG. 2, an initial condition of theexample operation of the arc-less MEMS based switching system 18 isillustrated. The MEMS switch 20 is depicted as starting in a firstclosed state. Also, as indicated, there is a load current I_(LOAD) 50which has a value substantially equal to V_(BUS)/R_(LOAD) in the loadcircuit 40.

Moreover, for discussion of this example operation of the arc-less MEMSbased switching system 18, it may be assumed that a resistanceassociated with the MEMS switch 20 is sufficiently small such that thevoltage produced by the load current through the resistance of MEMSswitch 20 has only a negligible effect on the near-zero voltagedifference between the mid-points of the diode bridge 28 when pulsed.For example, the resistance associated with the MEMS switch 20 may beassumed to be sufficiently small so as to produce a voltage drop of lessthan a few millivolts due to the maximum anticipated load current.

It may be noted that in this initial condition of the MEMS basedswitching system 18, the pulse switch 54 is in a first open state.Additionally, there is no pulse circuit current in the pulse circuit 52.Also, in the pulse circuit 52, the capacitor C_(PULSE) 56 may bepre-charged to a voltage V_(PULSE), where V_(PULSE) is a voltage thatcan produce a half sinusoid of pulse current having a peak magnitudesignificantly greater (e.g., twice) the anticipated load currentI_(LOAD) 50 during the transfer interval of the load current. It may benoted that C_(PULSE) 56 and L_(PULSE) 58 may be selected so as toresonate with each other.

FIG. 3 illustrates a schematic diagram 64 depicting a process oftriggering the pulse circuit 52. It may be noted that detectioncircuitry (not shown) may be coupled to the pulse circuit 52. Thedetection circuitry may include sensing circuitry (not shown) configuredto sense a level of the load circuit current I_(LOAD) 50 and/or avoltage level of the voltage source V_(BUS) 44, for example.Furthermore, the detection circuitry may be configured to detect aswitch condition as described above. In one embodiment, the switchcondition may occur due to the current level and/or the voltage levelexceeding a predetermined threshold.

The pulse circuit 52 may be configured to detect the switch condition tofacilitate switching the present closed state of the MEMS switch 20 to asecond open state. In one embodiment, the switch condition may be afault condition generated due to a voltage level or load current in theload circuit 40 exceeding a predetermined threshold level. However, aswill be appreciated, the switch condition may also include monitoring aramp voltage to achieve a given system-dependent ON time for the MEMSswitch 20.

In one embodiment, the pulse switch 54 may generate a sinusoidal pulseresponsive to receiving a trigger signal as a result of a detectedswitching condition. The triggering of the pulse switch 54 may initiatea resonant sinusoidal current in the pulse circuit 52. The currentdirection of the pulse circuit current may be represented by referencenumerals 66 and 68. Furthermore, the current direction and relativemagnitude of the pulse circuit current through the first diode 30 andthe second diode 32 of the first branch 29 of the balanced diode bridge28 may be represented by current vectors 72 and 70 respectively.Similarly, current vectors 76 and 74 are representative of a currentdirection and relative magnitude of the pulse circuit current throughthe third diode 34 and the fourth diode 36 respectively.

The value of the peak sinusoidal bridge pulse current may be determinedby the initial voltage on the pulse capacitor C_(PULSE) 56, value of thepulse capacitor C_(PULSE) 56 and the value of the pulse inductanceL_(PULSE) 58. The values for the pulse inductance L_(PULSE) 58 and thepulse capacitor C_(PULSE) 56 also determine the pulse width of the halfsinusoid of pulse current. The bridge current pulse width may beadjusted to meet the system load current turn-off requirement predicatedupon the rate of change of the load current (V_(BUS)/L_(LOAD)) and thedesired peak let-through current during a load fault condition.According to aspects of the present invention, the pulse switch 54 maybe configured to be in a conducting state prior to opening the MEMSswitch 20.

It may be noted that triggering of the pulse switch 54 may includecontrolling a timing of the pulse circuit current I_(PULSE) 62 throughthe balanced diode bridge 28 to facilitate creating a lower impedancepath as compared to the impedance of a path through the contacts of theMEMS switch 20 during an opening interval. In addition, the pulse switch54 may be triggered such that a desired voltage drop is presented acrossthe contacts of the MEMS switch 20.

In one embodiment, the pulse switch 54 may be a solid-state switch thatmay be configured to have switching speeds in the range of nanosecondsto microseconds, for example. The switching speed of the pulse switch 54should be relatively fast compared to the anticipated rise time of theload current in a fault condition. The current rating of the MEMS switch20 is dependent on the rate of rise of the load current, which in turnis dependent on the inductance L_(LOAD) 46 and the bus supply voltageV_(BUS) 44 in the load circuit 40 as previously noted. The MEMS switch20 may be appropriately rated to handle a larger load current I_(LOAD)50 if the load current I_(LOAD) 50 may rise rapidly compared to thespeed capability of the bridge pulse circuit.

The pulse circuit current I_(PULSE) 62 increases from a value of zeroand divides equally between the first and second branches 29, 31 of thebalanced diode bridge 28. In accordance with one embodiment, thedifference in voltage drops across the branches 29, 31 of the balanceddiode bridge 28 may be designed to be negligible, as previouslydescribed. Further, as previously described, the diode bridge 28 isbalanced such that the voltage drop across the first and second branchesof the diode bridge 28 are substantially equal. Moreover, as theresistance of the MEMS switch 20 in a present closed state is relativelylow, there is a relatively small voltage drop across the MEMS switch 20.However, if the voltage drop across the MEMS switch 20 happened to belarger (e.g., due to an inherent design of the MEMS switch), thebalancing of the diode bridge 28 may be affected as the diode bridge 28is operatively coupled in parallel with the MEMS switch 20. Inaccordance with aspects of the present invention, if the resistance ofthe MEMS switch 20 causes a significant voltage drop across the MEMSswitch 20 then the diode bridge 28 may accommodate the resultingimbalance of the pulse bridge by increasing the magnitude of the peakbridge pulse current.

Referring now to FIG. 4, a schematic diagram 78 is illustrated in whichopening of the MEMS switch 20 is initiated. As previously noted, thepulse switch 54 in the pulse circuit 52 is triggered prior to openingthe MEMS switch 20. As the pulse current I_(PULSE) 62 increases, thevoltage across the pulse capacitor C_(PULSE) 56 decreases due to theresonant action of the pulse circuit 52. In the ON condition in whichthe switch is closed and conducting, the MEMS switch 20 presents a pathof relatively low impedance for the load circuit current I_(LOAD) 50.

Once the amplitude of the pulse circuit current I_(PULSE) 62 becomesgreater than the amplitude of the load circuit current I_(LOAD) 50(e.g., due to the resonant action of the pulse circuit 52), a voltageapplied to the gate contact 26 of the MEMS switch 20 may beappropriately biased to switch the present operating state of the MEMSswitch 20 from the first closed and conducting state to an increasingresistance condition in which the MEMS switch 20 starts to turn off(e.g., where the contacts are still closed but contact pressurediminishing due the switch opening process) which causes the switchresistance to increase which in turn causes the load current to start todivert from the MEMS switch 20 into the diode bridge 28.

In this present condition, the balanced diode bridge 28 presents a pathof relatively low impedance to the load circuit current I_(LOAD) 50 ascompared to a path through the MEMS switch 20, which now exhibits anincreasing contact resistance. It may be noted that this diversion ofload circuit current I_(LOAD) 50 through the MEMS switch 20 is anextremely fast process compared to the rate of change of the loadcircuit current I_(LOAD) 50. As previously noted, it may be desirablethat the values of inductances L₁ 84 and L₂ 88 associated withconnections between the MEMS switch 20 and the balanced diode bridge 28be very small to avoid inhibition of the fast current diversion.

The process of current transfer from the MEMS switch 20 to the pulsebridge continues to increase the current in the first diode 30 and thefourth diode 36 while simultaneously the current in the second diode 32and the third diode 34 diminish. The transfer process is completed whenthe mechanical contacts 22, 24 of the MEMS switch 20 are separated toform a physical gap and all of the load current is carried by the firstdiode 30 and the fourth diode 36.

Consequent to the load circuit current I_(LOAD) being diverted from theMEMS switch 20 to the diode bridge 28 in direction 86, an imbalanceforms across the first and second branches 29, 31 of the diode bridge28. Furthermore, as the pulse circuit current decays, voltage across thepulse capacitor C_(PULSE) 56 continues to reverse (e.g., acting as a“back electro-motive force”) which causes the eventual reduction of theload circuit current I_(LOAD) to zero. The second diode 32 and the thirddiode 34 in the diode bridge 28 become reverse biased which results inthe load circuit now including the pulse inductor L_(PULSE) 58 and thebridge pulse capacitor C_(PULSE) 56 and to become a series resonantcircuit.

Turning now to FIG. 5, a schematic diagram 94 for the circuit elementsconnected for the process of decreasing the load current is illustrated.As alluded to above, at the instant that the contacts of the MEMS switch20 part, infinite contact resistance is achieved. Furthermore, the diodebridge 28 no longer maintains a near-zero voltage across the contacts ofthe MEMS switch 20. Also, the load circuit current I_(LOAD) is now equalto the current through the first diode 30 and the fourth diode 36. Aspreviously noted, there is now no current through the second diode 32and the third diode 34 of the diode bridge 28.

Additionally, a significant switch contact voltage difference from thedrain 24 to the source 26 of the MEMS switch 20 may now rise to amaximum of approximately twice the V_(BUS) voltage at a rate determinedby the net resonant circuit which includes the pulse inductor L_(PULSE)58, the pulse capacitor C_(PULSE) 56, the load circuit inductor L_(LOAD)46, and damping due to the load resistor R_(LOAD) 48 and circuit losses.Moreover, the pulse circuit current I_(PULSE) 62, that is now equal tothe load circuit current I_(LOAD) 50, may resonantly decrease to a zerovalue and to maintain the zero value due to the reverse blocking actionof the diode bridge 28 and the diode D_(P) 60. The voltage across thepulse capacitor C_(PULSE) 56 has reversed resonantly to a negative peakand will maintain the negative peak value until the pulse capacitorC_(PULSE) 56 is recharged.

The diode bridge 28 may be configured to maintain a near-zero voltageacross the contacts of the MEMS switch 20 until the contacts separate toopen the MEMS switch 20, thereby preventing damage by suppressing anyarc that would tend to form between the contacts of the MEMS switch 20during opening. Additionally, the contacts of the MEMS switch 20approach the opened state at a much reduced contact current through theMEMS switch 20. Also, any stored energy in the circuit inductance, theload inductance and the source may be transferred to the pulse circuitcapacitor C_(PULSE) 56 and may be absorbed via voltage dissipationcircuitry (not shown). The voltage snubber circuit 33 may be configuredto limit voltage overshoot during the fast contact separation due to theinductive energy remaining in the interface inductance between thebridge and the MEMS switch. Furthermore, the rate of increase of reapplyvoltage across the contacts of the MEMS switch 20 during opening may becontrolled via use of the snubber circuit (not shown).

It may also be noted that although a gap is created between the contactsof the MEMS switch 20 when in an open state, a leakage current maynonetheless exist between the load circuit 40 and the diode bridgecircuit 28 around the MEMS switch 20. This leakage current may besuppressed via introduction of a secondary mechanical switch (not shown)series connected in the load circuit 40 to generate a physical gap. Incertain embodiments, the mechanical switch may include a second MEMSswitch.

FIG. 6 illustrates an exemplary embodiment 96 wherein the switchingcircuitry 12 (see FIG. 1) may include multiple MEMS switches arranged ina series or series-parallel array, for example. Additionally, asillustrated in FIG. 6, the MEMS switch 20 may replaced by a first set oftwo or more MEMS switches 98, 100 electrically coupled in a seriescircuit. In one embodiment, at least one of the first set of MEMSswitches 98, 100 may be further coupled in a parallel circuit, where theparallel circuit may include a second set of two or more MEMS switches(e.g., reference numerals 100, 102). In accordance with aspects of thepresent invention, a static grading resistor and a dynamic gradingcapacitor may be coupled in parallel with at least one of the first orsecond set of MEMS switches.

Referring now to FIG. 7, an exemplary embodiment 104 of a graded MEMSswitch circuit is depicted. The graded switch circuit 104 may include atleast one MEMS switch 106, a grading resistor 108, and a gradingcapacitor 110. The graded switch circuit 104 may include multiple MEMSswitches arranged in a series or series-parallel array as for exampleillustrated in FIG. 6. The grading resistor 108 may be coupled inparallel with at least one MEMS switch 106 to provide voltage gradingfor the switch array. In an exemplary embodiment, the grading resistor108 may be sized to provide adequate steady state voltage balancing(division) among the series switches while providing acceptable leakagefor the particular application. Furthermore, both the grading capacitor110 and grading resistor 108 may be provided in parallel with each MEMSswitch 106 of the array to provide sharing both dynamically duringswitching and statically in the OFF state. It may be noted thatadditional grading resistors or grading capacitors or both may be addedto each MEMS switch in the switch array.

FIG. 8 is a flow chart of exemplary logic 112 for switching a MEMS basedswitching system from a present operating state to a second state. Inaccordance with exemplary aspects of the present technique, a method forswitching is presented. As previously noted, detection circuitry may beoperatively coupled to the arc suppression circuitry and configured todetect a switch condition. In addition, the detection circuitry mayinclude sensing circuitry configured to sense a current level and/or avoltage level.

As indicated by block 114, a current level in a load circuit, such asthe load circuit 40 (see FIG. 2), and/or a voltage level may be sensed,via the sensing circuitry, for example. Additionally, as indicated bydecision block 116 a determination may be made as to whether either thesensed current level or the sensed voltage level varies from or exceedsan expected value. In one embodiment, a determination may be made (viathe detection circuitry, for example) as to whether the sensed currentlevel or the sensed voltage level exceeds respective predeterminedthreshold levels. Alternatively, voltage or current ramp rates may bemonitored to detect a switch condition without a fault having actuallyoccurred.

If the sensed current level or sensed voltage level varies or departsfrom an expected value, a switch condition may be generated as indicatedby block 118. As previously noted, the term “switch condition” refers toa condition that triggers changing a present operating state of the MEMSswitch. In certain embodiments, the switch condition may be generatedresponsive to a fault signal and may be employed to facilitateinitiating opening of the MEMS switch. It may be noted that blocks114-118 are representative of one example of generating a switchcondition. However as will be appreciated, other methods of generatingthe switch condition are also envisioned in accordance with aspects ofthe present invention.

As indicated by block 120, the pulse circuit may be triggered toinitiate a pulse circuit current responsive to the switch condition. Dueto the resonant action of the pulse circuit, the pulse circuit currentlevel may continue to increase. Due at least in part to the diode bridge28, a near-zero voltage drop may be maintained across the contacts ofthe MEMS switch if the instantaneous amplitude of the pulse circuitcurrent is significantly greater than the instantaneous amplitude of theload circuit current. Additionally, the load circuit current through theMEMS switch may be diverted from the MEMS switch to the pulse circuit asindicated by block 122. As previously noted, the diode bridge presents apath of relatively low impedance as opposed to a path through the MEMSswitch, where a relatively high impedance increases as the contacts ofthe MEMS switch start to part. The MEMS switch may then be opened in anarc-less manner as indicated by block 124.

As previously described, a near-zero voltage drop across contacts of theMEMS switch may be maintained as long as the instantaneous amplitude ofthe pulse circuit current is significantly greater than theinstantaneous amplitude of the load circuit current, therebyfacilitating opening of the MEMS switch and suppressing formation of anyarc across the contacts of the MEMS switch. Thus, as describedhereinabove, the MEMS switch may be opened at a near-zero voltagecondition across the contacts of the MEMS switch and with a greatlyreduced current through the MEMS switch.

FIG. 9 is a graphical representation 130 of experimental resultsrepresentative of switching a present operating state of the MEMS switchof the MEMS based switching system, in accordance with aspects of thepresent technique. As depicted in FIG. 9, a variation in amplitude 132is plotted against a variation in time 134. Also, reference numerals136, 138 and 140 are representative of a first section, a secondsection, and a third section of the graphical illustration 130.

Response curve 142 represents a variation of amplitude of the loadcircuit current as a function of time. A variation of amplitude of thepulse circuit current as a function of time is represented in responsecurve 144. In a similar fashion, a variation of amplitude of gatevoltage as a function of time is embodied in response curve 146.Response curve 148 represents a zero gate voltage reference, whileresponse curve 150 is the reference level for the load current prior toturn-off.

Additionally, reference numeral 152 represents region on the responsecurve 142 where the process of switch opening occurs. Similarly,reference numeral 154 represents a region on the response curve 142where the contacts of the MEMS switch have parted and the switch is inan open state. Also, as can be seen from the second section 138 of thegraphical representation 130, the gate voltage is pulled low tofacilitate initiating opening of the MEMS switch. Furthermore, as can beseen from the third section 140 of the graphical representation 130, theload circuit current 142 and the pulse circuit current 144 in theconducting half of the balanced diode bridge are decaying.

Additional aspects of the present invention comprise the addition of anenergy-absorbing circuitry 200, shown in FIG. 10, adapted to absorb(e.g., trap or take up) electrical energy during a switched load currentinterruption of a protected load circuit, such as may occur in responseto a fault condition. This circuitry is connected in a parallel circuitwith balanced diode bridge 28. It is contemplated that this additionwill enable optimization of load current interruption due to faultconditions that may develop in the load circuit and will be conducive toa decreased amount of let-through current in switching circuitryembodying aspects of the present invention. Moreover, aspects of thepresent invention should simplify the circuit design process, and shouldfurther enable optimization of circuitry components in a MEMS-basedswitching system, such as enabling appropriate circuitry selection for agiven application, and reduced weight and cost of the switching system.

In one example embodiment, energy-absorbing circuitry 200 is connectedacross the DC side of balanced diode bridge 28 in parallel with pulsecircuit 52 as shown in FIG. 10. Circuitry 200 may comprise a resistorRt, a diode Dt, and an energy-absorbing element, such as a capacitor Ct.For purposes of description of operational interactions effected bycircuitry 200, let us begin upon the opening of MEMS switch 20 to anon-conductive state, and upon a turning off of the two mutuallydiagonal bridge diodes (e.g., diodes 32 and 34) that may be sheddingcurrent during the process for limiting fault current. The diode bridgevoltage will rapidly rise above an initial voltage value (VtIni) storedon capacitor Ct and diode Dt will then turn on. It will be appreciatedthat electrical energy, which is stored in capacitor Ct upon occurrenceof a fault, may be readily discharged by way of a suitable dischargeresistor (not shown in FIG. 10).

The inventors of the present invention have innovatively recognizedstructural and/or operational relationships that permit independentoptimization of pulse circuit 52 and energy-absorbing circuitry 200 fora given circuit breaker application. This optimization can result in anappropriate selection of components and/or substantial cost reductionsfor a given application. For example, the value of pulse capacitor Cpand its initial voltage (VpIni) may be advantageously selected (e.g.,selecting a sufficiently small capacitance value for capacitor Cp) toachieve an optimum peak pulse current and/or pulse width independent ofenergy-absorbing capacitor Ct. That is, independent of energy trappingrequirements that otherwise would have to be carried out by way of pulsecapacitor Cpulse in addition to the pulse-forming requirements.Similarly, the value of energy-absorbing capacitor Ct and its initialvoltage (VtIni) can be independently selected (e.g., selecting asufficiently large capacitance value for energy-absorbing capacitor Ct)for rapid absorption of the fault energy during a fault interval. Thatis, independent of the pulse-forming requirements carried out by way ofcapacitor Cp.

A circuit arrangement embodying aspects of the present invention shouldresult in a reduced amount of fault let-through current and lowerbreaker energy dissipation. As noted above, diode bridge 28 and MEMSswitch 20 may be packaged to be closely integrated with one another(e.g., composite packaging) for reduction of parasitic inductance in thediode bridge and the respective interconnections to the MEMS switch.This incrementally reduces the amount of electrical energy that may bestored in such interconnections. Otherwise, during the opening of theMEMS switch, a corresponding incremental amount of electrical energywould have to be dissipated by the opening contacts of the MEMS switchsince the additional inductance of the interconnections to snubber 33would diminish effecting protective action by the protective circuitry(e.g., snubber 33) connected across MEMS switch 20 at the instant thecontacts disengage. It is noted that one important function of snubber33 is to retard (e.g., slow down) the rise of the voltage across theMEMS switch during the opening motion of the contacts to prevent arcingdue to excessive voltage gradient buildup until full separation andvoltage hold-off capability has been achieved.

In operation, (in response to a fault that can develop in the loadcircuit) reliable and substantially rapid turn-off of the load currentmay be accomplished through the following example sequence of events, asmay be performed in a MEMS-based switching system embodying aspects ofthe present invention: Capacitors Cpulse and Ct may be respectivelyinitially charged with the respective example voltage polarities shownin FIG. 10 to initial voltages VpInit and VtInit. MEMS switch 20initially will be in an ON (e.g., conductive) state in response to agating signal applied via contact gate 26. Upon occurrence of a fault,load current will rise at a relatively fast rate (e.g., high value ofdi/dt). For example, when the magnitude of the load current exceeds apredefined threshold value and/or the rate of change (di/dt) of the loadcurrent exceeds a predefined value, pulse switch 54 is triggered to anON state. The value of Ipulse current will then increase to a suitablepreset (design) value that exceeds the fault current value. Aspreviously discussed, the pulsed diode bridge 28 is configured to causea nearly zero voltage drop across MEMS switch 20. At a time just priorto reaching the peak value of Ipulse current, MEMS switch 20 will begated to an OFF (non-conductive) state.

As the MEMS switch contact pressure decreases in response to adecreasing gating signal, the increasing contact resistance forces acurrent transfer from the load circuit to the diode bridge 28. Currentflow through switch contacts 22 and 24 decreases to essentially zeroprior to the separation of these contacts. Contacts 22 and 24 will beopen under a nearly zero voltage condition. The fault current will nowbe carried by diode bridge 28. The value of Ipulse current will decrease(e.g., in a sinusoidal manner) after reaching its peak value. When thevalue of Ipulse current decreases to a value that matches theinstantaneous value of the increasing fault current, diodes D2 and D3will cease to conduct. At this point, diodes D1 and D4 will conduct thefault current from the load circuit. Also pulse capacitor Cpulsedecreases in voltage until its voltage polarity reverses. The increasein the magnitude of the reverse voltage being stored on capacitor Cpulseby the fault current will oppose bus source voltage 44 and will cause adecrease in the rise rate of fault current, di/dt.

The voltage across the DC input to diode bridge 28 will then increaserapidly and when its value reaches the initial voltage (VtInit) storedon capacitor Ct, diode Dt will begin to conduct and circuitry 200 willtake up the fault current. Capacitor Ct, as it is being charged, createsa reverse emf which reverses the rise rate of fault current (e.g.,negative di/dt). For instance, when the voltage on capacitor Ct reachesthe bus source voltage, then the fault current will decrease to zero.From the foregoing description, it will be appreciated thatenergy-absorbing capacitor Ct is adapted to absorb fault-resultingelectrical energy independent of the pulse circuit, and to cause theextremely rapid reduction of load current to zero through its resonantaction with the load and pulse inductances.

It will be appreciated that a circuit arrangement embodying aspects ofthe present invention advantageously separates the functions provided bycapacitors Ct and Cpulse. Furthermore, the circuit branch with theenergy-absorbing capacitor Ct may be conceptualized to function as a“snubber” in the sense of controlling (e.g., restraining) therate-of-change (e.g., rate-of-rise) of the voltage across the diodebridge as well as absorbing the load/source system energy. This may beachieved in one example embodiment with an adjustable pre-charge oncapacitor Ct.

FIG. 11 synthesizes in graphical form the operational concepts discussedabove through simulated circuit signals plotted as a function of timefor the MEMS-based switching system shown in FIG. 10. These signals areplotted over an example time interval of 10 microseconds (as may beuseful for appreciating operational details regarding an example initialtransient response of the switching system) during a switched turn-offof the load current in response to a fault condition while FIG. 12 showssuch simulated circuit signals plotted over a longer time interval, suchas 120 microseconds, (as may be useful for appreciating operationaldetails over that longer time interval and regarding an example responseof the switching system subsequent to turn-off of the load current).

The signal plots in FIGS. 11 and 12 are identified as follows inreference to the system shown in FIG. 10: Id1 and Id2 represent currentsrespectively conducted by bridge diodes D1 and D2. Ipulse representspulse current. Isw_in represents current flow through contacts 22 and 24of MEMS switch 20. Itrap represents current flow throughenergy-absorbing circuitry 200. Load Current represents a load currentin response to a fault condition, as such current is effectively limitedby the system shown in FIG. 10. V_Cpulse represents a voltage acrosspulse capacitor 56. Vbridge represents a voltage across diode bridge 28.Vd2 represents a voltage across diode D2. Vsw represents a voltageacross contacts 22 and 24 of MEMS switch 20. Vtrap represents a voltageacross energy-absorbing capacitor Ct.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A system comprising: a micro-electromechanical system switch; abalanced diode bridge configured to suppress arc formation betweencontacts of the micro-electromechanical system switch; a pulse circuitcoupled to the balanced diode bridge, the pulse circuit comprising apulse capacitor adapted to form a pulse signal for causing flow of apulse current through the balanced diode bridge, the pulse signal beinggenerated in response to a fault condition in a load circuit coupled tothe micro-electromechanical system switch; energy-absorbing circuitrycoupled in a parallel circuit with the pulse circuit, theenergy-absorbing circuitry comprising an energy-absorbing capacitor anda diode connected in series, the energy-absorbing capacitor beingcharged to store an initial voltage value prior to the fault conditionand the diode being so connected as to be in a reverse-biased state whenthe energy-absorbing capacitor is charged to store the initial voltagevalue, and in a conductive state when a voltage developed across thediode bridge reaches the initial voltage value stored in theenergy-absorbing capacitor; and wherein the energy-absorbing capacitoris further adapted to absorb electrical energy resulting from the faultcondition and not affect pulse signal formation by the pulse circuit,and the conductive state of the diode in the energy-absorbing circuitrycauses the energy absorbing capacitor to receive fault current onlyafter the voltage across the diode bridge surpasses the initial voltagevalue.
 2. The system of claim 1 wherein a capacitance value of the pulsecapacitor is selected to control one or more pulse signalcharacteristics of the pulse signal independent of a capacitance valueof the energy-absorbing capacitor.
 3. The system of claim 2 wherein theone or more pulse signal characteristics of the pulse signal areselected from the group consisting of a width of the pulse signal, apeak of the pulse signal, and a combination of the width and the peak ofthe pulse signal.
 4. The system of claim 1 wherein a capacitance valueof the energy-absorbing capacitor is selected to control an amount ofelectrical energy absorbed by the energy-absorbing capacitor independentof a capacitance value of the pulse capacitor.
 5. The system of claim 1,wherein the balanced diode bridge comprises a first branch and a secondbranch, and wherein the first branch comprises a first diode and asecond diode coupled in a first series circuit and the second branchcomprises a third diode and a fourth diode coupled in a second seriescircuit.
 6. The system of claim 5, wherein the micro-electromechanicalsystem switch is coupled in parallel across midpoints of the balanceddiode bridge, and wherein a first midpoint is located between the firstand second diodes and a second midpoint is located between the third andfourth diodes.
 7. The system of claim 1, wherein themicro-electromechanical system switch is integrated with the balanceddiode bridge in a single package.
 8. The system of claim 1, wherein thepulse circuit is further configured to detect a fault condition andinitiate opening of the micro-electromechanical system switch responsiveto the fault condition.
 9. The system of claim 1, further comprising afirst plurality of micro-electromechanical switches electrically coupledin a series circuit.
 10. The system of claim 9, wherein at least one ofthe first plurality of micro-electromechanical switches is furthercoupled in a parallel circuit comprising a second plurality ofmicro-electromechanical switches.
 11. A system comprising: switchingcircuitry comprising a micro-electromechanical system switch configuredto switch the system from a first switching state to a second switchingstate; arc suppression circuitry coupled to the switching circuitry,wherein the arc suppression circuitry is configured to suppress an arcformation between contacts of the micro-electromechanical system switch;detection circuitry coupled to the arc suppression circuitry andconfigured to determine existence of a fault condition of an electricalload circuit coupled to the switching circuitry; a pulse circuit coupledto the arc suppression circuitry and the detection circuitry, whereinthe pulse circuit is configured to form a pulse signal responsive to thefault condition, and wherein the pulse signal is applied to the arcsuppression circuitry in connection with initiating an opening of themicro-electromechanical system switch; energy-absorbing circuitrycoupled in a parallel circuit with the pulse circuit, theenergy-absorbing circuitry comprising an energy-absorbing capacitor anda diode connected in series, the energy-absorbing capacitor beingcharged to store an initial voltage value prior to the fault conditionand said diode being so connected as to be in a reverse-biased statewhen the energy-absorbing capacitor is charged to store an initialvoltage value, and in a conductive state when a voltage developed acrossthe arc suppression circuitry reaches the initial voltage value storedin the energy-absorbing capacitor; and wherein the energy-absorbingcapacitor is further adapted to absorb electrical energy resulting fromthe fault condition and not affect a pulse signal formation by the pulsecircuit, and the conductive state of the diode in the energy-absorbingcircuitry causes the energy absorbing capacitor to receive fault currentonly after the voltage across the diode bridge surpasses the initialvoltage value.
 12. The system of claim 11, wherein the arc suppressioncircuitry comprises a balanced diode bridge coupled in a parallelcircuit with the micro-electromechanical system switch.
 13. The systemof claim 11, wherein the pulse circuit comprises a pulse capacitor thataffects one or more characteristics of the pulse signal.
 14. The systemof claim 13 wherein a capacitance value of the energy-absorbingcapacitor is selected to control an amount of electrical energy absorbedby the energy-absorbing capacitor independent of a capacitance value ofthe pulse capacitor.
 15. The system of claim 14 wherein a capacitancevalue of the pulse capacitor is selected to control the one or morepulse signal characteristics of the pulse signal independent of thecapacitance value of the energy-absorbing capacitor.
 16. The system ofclaim 15 wherein the one or more pulse signal characteristics of thepulse signal are selected from the group consisting of a width of thepulse signal, a peak of the pulse signal, and a combination of the widthand the peak of the pulse signal.
 17. The system of claim 1, wherein theenergy-absorbing capacitor is further adapted to receive fault currentso as to charge and create a reverse emf to reverse the rate of rise offault current when the diode in energy-absorbing circuitry is in theconductive state.
 18. The system of claim 11, wherein theenergy-absorbing capacitor is further adapted to receive fault currentso as to charge and create a reverse emf to reverse the rate of rise offault current when the diode in energy-absorbing circuitry is in theconductive state.
 19. The system of claim 1, wherein the pulse capacitoris configured to decrease a rate of rise of fault current.
 20. Thesystem of claim 13, wherein the pulse capacitor decreases a rate of riseof fault current associated with the fault condition.